Toner is a developer material for a laser printer or a copy. The operation of a printer is substantially as follows: after exposure, an electrostatic latent image can be formed on the surface of a photosensitive drum charged uniformly. The toner charged by a developing roller and a blade can develop the electrostatic latent image on the surface of the photosensitive drum to form an image. Attracted by opposite charges on the surface of a paper, the image on the surface of photosensitive drum can be transferred and printed on the paper. Then we will get a desired printed image fixed by a heating roller and a pressure roller.
It can be seen from the printing process described above that the toner is charged by the developer roller and the blade, and the charges are obtained by the surface of the toner. The charged toner can develop on the surface of the photosensitive drum to form a pattern to be printed. In order to enable the toner to have a good chargeability, a charge control agent (CCA) is required to be dispersed on the surface of the toner particles as uniformly as possible. Before being transferred and printed from the photosensitive drum to the paper, the toner is required to hold charged state continuously. If the charging characteristic and charge retention capacity of the toner is not good, it is difficult to develop or transfer print. It is also difficult to obtain the image with desired printing effect.
Similarly, the toner should have a number of other properties, such as excellent transfer printing property, low-temperature fixation and storage stability. The toner with a good sphericity has a high transfer printing efficiency, which can be easily transferred and printed from the photosensitive drum to the paper, while reducing or preventing toner remained on the surface of the photosensitive drum. To reduce the environmental impact, toner should fix on the paper at a temperature as low as possible. At the same time the toner should have good storage stability without melt adhesion even exposed to a high temperature condition during being handled or transported. The toner of core-shell structure improves its fixation property by a core with a low glass-transition temperature (low Tg), and enhances blocking resistance by a shell with a high glass-transition temperature (high Tg). By employing such a layer structure, it is possible to meet requirements of both low-temperature fixation and storage stability.
Traditional mechanical pulverizing toner can be obtained by melting and blending resin, CCA, pigment and wax, pulverizing and classifying. Since mechanical pulverizing toner has an irregular shape, the pressure applied by the blade to each toner particles varies, at the same time CCA tends to form a phase separated from a binder resin. Further, CCA particles exposed on the surface of the toner can easily fall off. Therefore, mechanical pulverizing toner is non-uniformly charged and has a very wide charge distribution.
Chinese Patent CN101427186 provides a mechanical pulverizing toner based on styrene/acrylate and polyester binder resin, using a charge control agent of styrene/acrylate which has a good compatibility with the toner. Therefore, the distribution uniformity and retention capacity of charges of the toner has been greatly improved. But it is difficult to meet requirements of low-temperature fixation. Further, the process in which toner particles are collided with one another for 5-20 minutes at 75 to 100 m/s linear velocity to carry out the process of spheroidization consumes a large amount of energy and has high requirement for equipments. Furthermore, it is difficult to obtain a positive spherical toner.
Compared with traditional mechanical pulverizing toners, the conventional suspension polymerization toner can be obtained by dispersing CCA and waxes, pigments and other components together into a monomer uniformly, and carrying out polymerization after high-speed shearing and granulation. The CCA in the toner particles disperses uniformly, and the toner particles have a good sphericity. Thereby, it has a good uniformity of charge distribution and transfer printing efficiency. But the method can still result in a significant amount of CCA distributed in the central region of the toner particles. Charges of the toner can be generated by the friction between the blade and the CCA on the surface of the toner. However the CCA dispersing in the central region of the toner particles does not involve in the friction, it can result in an inefficient use of the CCA. Therefore, there is a need to develop an effective method for increasing the charge density on the surface of the toner.
Chinese Patent CN101473274 provides a toner of core-shell structure which includes a rigid and thin shell layer with dense CCA distributing formed by coating the surface of toner core particles which is prepared by a suspension polymerization process with CCA, using a polymer formed by polymerization reaction of the shell monomer components. The method demands CCA particles being used should be carried out grind and dispersion treatment, and demands the particle diameter distribution thereof should be tightly controlled. It results in a large number of CCA dissociating because of a poor compatibility of CCA and shell resin.
Chinese Patent CN101727031 provides a polymerized toner with high dense charges on the surface which can be obtained by adding a reactive anionic surfactant which contains at least one reactive functional group into aqueous dispersion liquid or adding it at the initial stage of a suspension polymerization reaction, grafting to the surface of the toner particles via polymerization. If the amount of reactive anionic surfactants used in this method is too high, it can result in producing too fine particles in the suspension system, which causes background pollution on the printed images.